ioo THE BOOK OF NATURE STUDY 



We must note in the first instance that neither the tadpole 

 nor the frog is a powerful swimmer, neither can make continued 

 progress against a strong current. Therefore the animals are 

 necessarily inhabitants of pools and not streams. Now, if we 

 examine any region of pools and marshes in summer, we shall 

 find that these show a strong tendency to dry up. Very often, 

 indeed, one may see such ponds drying up before the tadpoles 

 have had time to run through their development, and in this 

 case we may see hundreds or thousands of the little creatures 

 dying of drought. One view, then, of the metamorphosis of the 

 tadpole is that it is an adaptation to obviate the risk of drought 

 in summer. In Africa there is an interesting fish called Pro- 

 topterus which inhabits marshy regions, where there is a similar 

 danger of drought at certain periods. The true home of Pro- 

 topterus, indeed, is the overflow regions of the tropical rivers, 

 the regions which are lakes after the rains, and dry land or swamp 

 in the dry season. Now Protopterus has solved its problem in 

 this way : in the wet season it is a true fish, breathing by gills, 

 swimming by paired fins, and having the circulation of a fish. 

 When the dry period comes, it buries itself in the mud to sleep 

 till the rains. But in its mud-case it could not breathe by its 

 gills, and it has for use at this period a pair of lungs, by means 

 of which it breathes. But the tadpole is in one respect worse 

 off than Protopterus, for it has to provide for a cold period as 

 well as for a period of drought. It could not sleep through both 

 periods. Therefore as summer comes to its height it develops 

 legs ending in feet and toes, it loses its gills, it becomes an air- 

 breathing, terrestrial animal, only returning to its pond to sleep 

 through the winter, and, as it becomes mature, to breed. From 

 one point of view, then, the metamorphosis of the frog may be 

 said to be an adaptation to obviate the danger of drought. 



Once again, because of the existence of strange transitional 

 fishes such as Protopterus in Africa, as well as for certain structural 

 reasons, zoologists believe that the frog has been descended 

 from a fish-like ancestor. The rocks tell us that we can look 

 back to a period when the fishes were the highest living animals, 

 and from some type of fish the first Amphibian arose. Now, it 

 is to the zoologist a fact of great importance that in many points 



